The constant progress of integration technologies has permitted the integration of an output power stage on the same chip containing the driving circuit and the circuit for controlling the operation of the output power device. By virtue of constant improvement of the technology, the level of current that may be delivered by an integrated power stage is continuously increasing. As a consequence, the diameter of the bond-wire used for connecting a pad of the integrated circuit with the respective lead of the package has also increased up to the limit of compatibility with the peculiar bonding techniques that are used for welding the wire on the metallized pad of the integrated circuit and on the inner end of the relative lead or pin of the package.
With the growing of design currents above the limit that can be safely handled by a single bond-wire of a relatively augmented diameter notwithstanding (especially for the nominal short circuit current), it becomes necessary to employ a second or a plurality of bond-wires in parallel with each other. A second bond-wire may be connected, as the first one, to a single pad of sufficient size, as shown in the diagram of FIG. 1. This solution has the drawback that in case of rupture of one or more bond-wires between the pad and the relative lead of the package, for example during the critical resin encapsulation phase of fabrication, the routine test of the end product will not detect the rupture, and during normal operation, the device may be subject to failure in case of overcurrent.
To obviate this problem, a known technique is to subdivide the power device into n modular devices, functionally driven in parallel with each other (where the number n may depend on the design current). Each current terminal of each modular power element may be connected to a respective pad, which is then connectable through a bond-wire to a single metal lead or pin of the package. The n pads altogether constitute a current terminal of the modularly integrated power device.
During final testing of the finished product, the modular power element is turned on and its series resistance is measured. An anomalous resistance value, higher than the expected value of the overall series resistance, will indicate the interruption of one or more of the n current paths, most probably due to the rupture of the relative bond-wire.
On the other hand, most output power stages have special control circuits which, for example, may include a regulator circuit of the output voltage. The voltage regulator generally employs an integrated voltage divider (R1-R2) connected to an output pad to sense an output voltage, as depicted in FIG. 3.
In case of rupture of the bond-wire that connects the pad through which the sensing voltage divider is connected to the relative lead, the voltage regulator will react as if the output voltage would be low, and therefore, will bring the output of the regulator to the input voltage.
This event has a dramatic effect in the case of a regulator for an alternator where, as a consequence of the behaviour of the voltage regulator circuit, the excitation will no longer be interrupted, thus causing the destruction of the battery.
In order to obviate to this risk, a known solution is to realize more sensing voltage dividers (one for each output pad), and as many loops or regulating circuits and a logic OR circuit capable of interrupting field excitation when anyone of the regulating loops rises above a reference voltage.
As schematically depicted in FIG. 4, this approach is used in many known systems, wherein the realization of a pair of resistances, an operational amplifier, and a capacitance of about 1 nanofard, necessary for implementing each regulating loop, do not materially burden the cost of the overall system. On the other hand, in many fully integrated systems, a solution of this type may be extremely burdensome and substantially inapplicable.